1. Field of the Invention
The present invention relates to a motor vehicle tyre with an antiabrasive band which is capable of reducing its rolling resistance (R.R.).
2. Description of the Related Art
One of the most urgent requirements on the part of motor vehicle constructors regarding the performance qualities of tyres is that of a low R.R.
As a result of this requirement, tyre constructors have hitherto made strenuous efforts to reduce the R.R. without negatively affecting other important characteristics such as the handling, the comfort and the durability.
Among the various components which constitute a tyre, the one which most greatly influences the R.R. is clearly the tread band, since it is the component which comes into direct contact with the ground.
Thus, in recent years, efforts of experts in the field have been concentrated on modifications to the tread compound. In particular, it has been sought to modify the hysteresis properties of the tread compounds, in particular by varying the loss factor, which is defined as tan δ=E″/E′, in which E′=storage modulus and E″=loss modulus. This is because it is thought that an optimum compromise between a low R.R. and good road holding of the tyre on wet surfaces would be obtained with a compound having a low value of tan δ at moderate temperatures (50–70° C.) and a high value of tan δ at low temperatures (0–10° C.).
In this context, a particularly significant change in the hysteresis behaviour of the compounds was obtained by totally or partially replacing the conventional reinforcing fillers based on carbon black with so-called “white fillers” and in particular with silica (see, for example, patent EP-501 227).
Besides this, attempts have been made to improve the R.R. by modifying the compound used for the tread underlayer and for the carcass plies or for the side walls of the tyre.
U.S. Pat. No. 4,319,619 describes a radial tyre in which the rubber portion of at least one carcass ply and of the tread underlayer are made of a rubber which has a viscoelastic property tan δ≦0.2 and a storage modulus≧120 Kg/cm2. In this tyre, the R.R. would be reduced without degrading the handling in terms of braking, stability, comfort and wear resistance.
According to U.S. Pat. No. 5,929,157, the R.R. of a tyre can be reduced without reducing the wear resistance or the handling on wet ground and without increasing the electrical resistance of the tyre as a whole if the compound for the side wall of the tyre is produced using, as reinforcing filler, a particular type of carbon black, replacing some of the carbon black with a particular type of silica and, particularly, using a specific amount of a silane as coupling agent (col. 1, lines 32–39).
It is commonly perceived that the hysteresis properties of other parts of the tyre do not have a particularly significant influence on the overall R.R. of the tyre. This view is confirmed by calculations on models produced by analysis of the finished components [see, for example, J. L. Locatelli and Y. De Puydt in “Tire Technology International” (June 1999, pp. 50–55)].
Changing other parts of the tyre to reduce the R.R. is moreover discouraged by the fact that obtaining only a modest improvement in the R.R. entails running the risk of compromising other important characteristics and, thus, of impairing the overall performance qualities of the tyre.
This is particularly true for the antiabrasive band, which needs to have properties that are in direct opposition to the changes which may be envisaged for reducing the R.R. The reason for this is that, as is known, the antiabrasive band is a component made of elastomeric material which forms part of the tyre bead and is placed in a position axially external to the carcass fold. Once the tyre is mounted on the wheel rim, the antiabrasive band then lies between the carcass fold and the rim and serves mainly to protect the carcass fold from the continuous deformational and frictional stresses exerted by the rim on the bead.
Considering its position and its function, it conventionally consists of a compound whose storage modulus is high enough not to make it a mechanically heterogeneous component relative to the bead. At the same time, this compound must also have wear resistance and fatigue strength values which are such that they allow the antiabrasive band to exercise the abovementioned protective function efficiently.
In general, an antiabrasive band compound comprises 30–70 phr of natural rubber, 30–70 phr of BR (polybutadiene), 0–20 phr of SBR (styrene/butadiene copolymer), at least 80 phr of carbon black, a large amount of sulphur (at least 2 phr) and a large amount of oil (about 15 phr). The combined use of large amounts of carbon black and sulphur gives the compound high storage modulus E′ values at high temperatures (70 and 100° C.).
In the course of the present description and in the claims which follow, the expression phr (per hundred rubber) means that the weights of the various components of the compound are relative to 100 parts of rubber.
Typically, an antiabrasive band compound conventionally has the following physical properties: modulus E′ at 70° C.=7.5–8.5 MPa; tan δ at 70° C.=0.15–0.20; DIN abrasion≦30 mm3.